This invention relates in general to vehicular braking systems and, in particular, to an improved method of traction control using the vehicular braking system.
Vehicles are equipped with a brake system for slowing or stopping movement of the vehicle in a controlled manner. A typical brake system for an automobile or light truck includes a disc brake assembly for each of the front wheels and either a drum brake assembly or a disc brake assembly for each of the rear wheels. The brake assemblies are actuated by hydraulic or pneumatic pressure generated when an operator of the vehicle depresses a brake pedal.
Advances in braking technology have led to the introduction of anti-lock braking systems (ABS). An ABS system monitors wheel rotational behavior and selectively applies and relieves brake pressure in the corresponding wheel brakes in order to maintain the wheel speed within a selected slip range to achieve maximum braking force. While such systems are typically adapted to control braking forces applied to each wheel of the vehicle, some systems have been developed for controlling the braking of only a portion of the plurality of braked wheels.
Electronically controlled ABS valves, which include apply valves and dump valves, are located between the master cylinder and the wheel brakes. The ABS valves regulate the pressure between the master cylinder and the wheel brakes. Typically, when activated, these ABS valves operate in three pressure control modes: pressure apply, pressure dump, and pressure hold. The apply valves allow pressurized brake fluid to flow into respective wheel brakes to increase pressure during the apply mode. The dump valves relieve brake fluid pressure from their associated wheel brakes during the dump mode. Wheel brake pressure is held constant during the hold mode by closing both the apply valves and the dump valves.
A further development in braking technology has led to the introduction of traction control systems. Typically, valves have been added to existing ABS systems to provide a brake system which controls wheel speed during acceleration. Excessive wheel speed during vehicle acceleration leads to wheel slippage and a loss of traction. A traction controller module detects this condition—i.e., a need for traction control—and automatically applies braking pressure to the wheel cylinders of the slipping wheel to reduce the slippage and increase the traction available. In order to achieve optimal vehicle acceleration, pressurized brake fluid is made available to the wheel cylinders even if the master cylinder is not actuated by the driver. Such a traction control system may be as disclosed in U.S. Patent Publication No. 2014/0131154 to Ganzel, the disclosure of which is incorporated herein by reference in entirety.
To detect a need for traction control due to excessive wheel speed during vehicle acceleration, a rotational speed of the wheel may be measured using a rotational speed sensor. However, data from wheel speed sensors do not permit the traction controller module to preemptively judge vehicle operating parameters. Furthermore, the rotational speed sensor may experience oscillations in the rotational speed of the wheel. Therefore, it would be desirable to provide a method of traction control that does not rely exclusively on rotational speed sensor data.